1. Field of the Invention
The present invention relates to an image pickup device for converting an image to an electrical signal, and more particularly, to an active pixel sensor (hereafter referred to as APS) having an amplifier for each pixel or several pixels.
2. Related Background Art
An image pickup device serving as an APS is also referred to as a CMOS sensor and widely used for digital cameras. This conventional image pickup device is described below by referring to FIGS. 9 to 12.
FIG. 9 is a top layout drawing pf pixels of a conventional image pickup device.
FIG. 10 is a cross-sectional structural drawing along the line 10-10 in FIG. 9.
FIG. 11 is a concentration profile along the line Y-Y in FIG. 10.
FIG. 12 is an equivalent circuit diagram using the image pickup device in FIG. 9.
In FIG. 12, reference numeral 121 denotes a photodiode (PD) for converting light into a signal charge and 122 and others denote a processing circuit for driving a PD or processing the signal charge of the PD 121. Reference numeral 122 denotes a transfer MOS transistor for transferring a signal charge generated by a photodiode, 123 denotes a floating diffusion (FD) region for temporarily storing the transferred signal charge, 124 denotes a resetting MOS transistor for resetting the floating diffusion region 123 and PD 121, 125 denotes a source follower MOS transistor for converting the signal charge of the floating diffusion region 123 into a voltage and amplifying the voltage by a source follower amplifier, 126 denotes a selection MOS transistor for selecting an optional one row in an array and 127 denotes a read line commonized by one column to read a pixel voltage signal. Pixels are arranged like an array to constitute an image pickup device.
In FIGS. 9 and 10, a PD of a photoelectric conversion unit is constituted of a P-type well region (hereafter referred to as “PWL”) 102 and a PN junction in an N region 103 on an N-type substrate 101. Reference numeral 107 denotes an N+ region serving as a source-drain region of a source follower MOS transistor which is a part of a circuit for amplifying a signal charge generated by a PD.
In the case of the APS of a prior art, the N region 103 of a PD serving as a photoelectric conversion unit is one of large sources for respectively generating a dark current. A dark current component generated nearby a separation layer 105 on the fringe of the N region 103 is present in addition of a component generated on the entire surface of the N region 103. Therefore, the N region 103 of the PD is separated from the separation layer 105 which is a generation region of the dark current component by a certain distance. A P+ region (channel stop region) 104 having a high P-type impurity concentration is formed between the separation layer 105 and the N region of the PD in order not to make a depletion layer extending from the N region 103 of the PD reach the separation layer. An example of the configuration is disclosed in Japanese Patent Application Laid-Open No. H10-308507. Moreover, in US2003160295A, it is reported that a dark current is extremely generated under a poly-wiring on a separation layer. The channel stop region 104 raises a P-type concentration of a channel stop region under a separation layer in which a poly-wiring (or poly-gate) 106 is set and realizes an effect for restraining a dark current by forming a concentration barrier on a path to the N region 103.
However, even by forming the channel stop region, when deepening a concentration profile of a PWL and using a PWL structure for improving a quantum efficiency of photoelectric conversion by a PD, a new technical problem is found that a dark current increases.
Specifically, as shown in FIG. 11, a dark current increases at a rate of more than 10% by changing a concentration profile of PWL1 102-1 to PWL2 102-2. Moreover, by setting a profile of PWL3 102-3, the dark current increases at a rate of tens of percents. This is a phenomenon which cannot be explained by a conventional idea that a dark current is generated because a depletion layer contacts with a separation layer. Moreover, when a well is deepened, a problem further occurs that the number of charges leaked from adjacent pixels increases. Specifically, a problem of deterioration of a blooming phenomenon or color mixture occurs.
Therefore, it is an object of the present invention to greatly restrain a dark current, blooming and color mixture when the concentration profile of the PWL is deep and a PWL structure for improving a quantum efficiency of photoelectric conversion by a PD is used.